1. Field of the Invention
The present invention relates to obtain on a display a curve of luminous flux corresponding to a photographic film when using an image sensor formed by a CCD member (Charge Coupled Device) instead of x-ray film for dental x-raying and more exactly to a method for compensation of dark current in the use of a CCD member for dental x-raying due to the CCD member having a strongly temperature and integration time dependent dark signal.
2. Description of the Related Art
In dental x-ray examinations small pieces of photographic film encapsuled in what could be called a cover to protect the film from light have for a long time been used, whereby said cover was being brought into the oral cavity. This film particularly adapted for exposure by x-ray radiation is in darkness developed after exposure and removal of the cover in a normal way, whereby an image is obtained where e.g. the jawbone and teeth will appear lighter in the film than for example soft tissues due to the difference in material density.
Today according to prior art the photographic film is replaced by electronic image sensors, usually in form of an image sensor of a CCD type (Charge Coupled Device). Such image sensing members are also frequently to be found in e.g. video technique but are then meant to especially operate within the wavelength range of visual light. The technique implies that immediately after the exposure with x-ray radiation a corresponding image is obtained for example on some type of display and thereby one avoids all of the developing process simultaneously as the radiation dose in most cases essentially is decreased due to that the image sensor itself may sense the necessary radiation dose to obtain the sufficient exposure. Such a system for dental x-raying named SEN-A-RAY is marketed for example by Regam Medical Systems AB, Sundsvall. When these images produced by the previously used dental x-ray film were evaluated, this was done by permitting light from behind to fall through the obtained image. The darkening of the film may be expressed by the optical density D as a function of the light intensity I.sub.1 reaching the eye relative to the light intensity I.sub.0 falling in. EQU D=(optical density)=10 log I.sub.0 /I.sub.1
In a dental film, D is basically proportional to the exposure dose. If one thereby, according to FIG. 2, draws a graph of the light intensity as a function of the optical density D, a nonlinear curve having the largest slope at high material densities of the object within the interval marked by a in FIG. 2 is obtained, i.e. the areas in the exposed film corresponding to enamel and dentine. Thus it desirable that the intensity curve within this density range be as steep as possible to be able to discriminate small changes in e.g. the enamel of a tooth.
In the case of a CCD sensor an intensity value is obtained, which is proportional to the exposure, and primarily the film and the sensor in this respect have equal qualities. Upon the presentation of the signal from such a CCD sensor, a visualization is generally obtained where the relation between intensity of light on the display and D will be a linear function, as is exemplified in FIG. 3. But simultaneously as a high resolution of gray levels within the area comprising e.g. the enamel of a tooth is desired, also still a resolution is desired at high optical densities corresponding to portions more opaque to the x-ray radiation, to e.g. clearly be able to indicate an eventual root infection and the like. This is why the problem is not simply solved only by making the linear function steep within a limited density interval. Accordingly an image having the linear intensity imaging does not to the user appear quite similar compared to film images and may sometimes result in certain interpretation difficulties. This is why it is desirable to be able to present image data also on a video monitor having the logarithmic or non-linear curve of FIG. 2.
By utilization of different technical methods it is of course possible by hardware or software to transform the image from the CCD sensor presented at the display screen to have a corresponding non-linear relation between the intensity of light an the density of the imaged object. By such a method a transfer function may be obtained which offers a steep slope i corresponding to the y-direction within a certain desired interval along the x-axis of the graph.
Anyhow, one difficulty in this case is to be able to ensure that the steep portion of the curve really falls within the interesting influenced interval, as the reference point or starting point of the curve is not fixed because the dark current of the CCD sensor is highly dependent upon e.g. the surrounding temperature and the integration time as it is necessary to operate in a temperature interval between about 20.degree. C. and 37.degree. C., whereby the temperature of the sensor will vary between a temperature higher than room temperature and higher than the body temperature of 37.degree. C. due to its own power dissipation and dependent on how much heat for example is transferred to the CCD cell within the oral cavity during the preparations before and also during the exposure and which at each different occasion for example is dependent of how the encapsuling of the CCD device transmits the environment temperature as well as how it makes contact to tissue and how exhalation air will affect the sensor. The necessary dose and consequently also the integration time will also vary from exposure to exposure because different objects require more or less dose to be penetrated. Thus, the integral of the dark current, the dark signal will be strongly dependent on both temperature and integration time.
In document U.S. Pat. No. 4,628,357 is disclosed a digital fluorographic system for the display of images of objects wherein by base line clipping an improvement is affected in the circuitry for log amplifying the video signal from a camera. However this is not intended for a single direct frame from a CCD cell, and utilizing fixed settings and clipping will not be a proper way to handle varying dark signal to obtain maximum sensitivity for faintly exposed areas of an image.
In another document U.S. Pat. No. 4,467,351 is disclosed a way to decrease the dose by deliberately underexposing the image on a film, an X-ray intensifier or the like and by means of a television camera enhancing the contrast by means of a digital imaging processor. The television camera then operates in a normal mode and does not produce only one single frame and here there are no measures taken regarding a possible dark signal which is highly dependent of the operating temperature as well as the integration time, which in the case of a CCD sensor for dental x-ray will be in a critically important range.
Thus, there is an obvious need to be able to control the influence of the dark signal on the starting point of the graph within the most interesting interval (marked by a in FIG. 2) of the contrast curve shown in FIG. 2 in the use of a CCD sensor. According to prior art this should normally be created by a simple transformation of the already digitized image signal by any suitable transfer function and eventually by means of a fixed compensation. A much better way should be to already from start dynamically depending on a precalculated dark signal component influence and lock the image signal within the correct amplitude interval before digitalization and thereby facilitate a further improved dynamics within the range of interest